1. Field of the Invention
The present invention relates to a polychrometer for measuring a spectral intensity distribution of a measurement light to be measured and a spectral reflection characteristic of a measurement sample to be measured, and a method for correcting stray light.
2. Description of the Background Art
A polychrometer for simultaneously measuring spectral intensities at all wavelengths of a measurement wavelength range is widely used as spectral means of a spectrophotometer (spectral intensity measuring apparatus) for measuring a spectral intensity distribution of a measurement light and a spectrocolorimeter (spectral reflection characteristic measuring apparatus) for measuring a spectral reflection characteristic of a sample because of its features of having high measurement efficiency and being able to measure an instant light. FIG. 6 is a section showing a schematic construction of a standard polychrometer 1. A beam I incident on an incident slit from a light source S is converted into a parallel beam by a lens L to be incident on a diffraction grating G, thereby being diffused and reflected, and a diffused image (diffused image by 1st-order diffracted lights D in the example of FIG. 6) of the incident slit SL is formed and gathered on a sensor array DA by the lens L. The sensor array DA includes, for example, 35 pixels having pixel numbers n=1 to 35 and covers a wavelength range of 380 to 720 nm.
In such a polychrometer, stray light influence measurement accuracy. FIG. 11 shows the diffraction efficiency of a typical diffraction grating for 1st-order diffracted lights. This diffraction grating is set such that diffraction efficiency is higher at a short wavelength side since the sensitivities of silicon sensors such as CCDs or CMOSs used in the sensor array DA are higher at a long wavelength side. Thus, in the case of using this diffraction grating as the diffraction grating G of the polychrometer 1 shown in FIG. 6, 70% of the incident light is incident on the sensor array DA as 1st-order diffracted lights at 400 nm and 40% of the incident light is incident thereon as 1st-order diffracted lights at 700 nm, but diffracted lights other than the 1st-order diffracted lights become stray light. In other words, a ratio of a 700 nm component of the incident light to become stray light is about twice as large as a 400 nm component thereof.
Accordingly, in Japanese unexamined Patent Publications No. H11-30552 (D1) and No. H07-209082 (D2), an output distribution of a wavelength range distant from a center wavelength when a single-wavelength light is incident is used as a stray light distribution for an incident light having the wavelength of the single-wavelength light, matrix data indicating stray light distribution for incident lights of wavelengths in a measurement wavelength range is obtained beforehand from stray light distribution obtained for a plurality of single-wavelength lights, and a stray light distribution by an actual measurement light is estimated and corrected based on a spectral distribution of the actual measurement light and the matrix data.
With the above conventional technology, in order to obtain the matrix data, it is necessary to measure outputs at the respective wavelengths of the stray light range where the single-wavelength lights having low energy are incident at only small ratios. It takes time and cost to obtain necessary accuracy.